1. Field of the Invention
This invention relates to a cement dispersant, a method for the production thereof, and a cement composition using the cement dispersant. More particularly, it relates to a cement dispersant which improves the dispersibility of cement particles, heightens the flowability of a cement composition, prevents the heightened flowability of cement composition from degradation by aging, and entrains a suitable amount of air to impart perfect workability to the cement composition and ensure formation of a concrete with high strength, a method for the production thereof, and a cement composition using the cement dispersant.
2. Description of the Prior Art
Since the early deterioration of concrete structures raised a serious social issue in 1981, the desirability of decreasing the unit water content of concrete and enhancing the workability and durability of concrete has been finding enthusiastic recognition. This has led to considerable work in the development of cement dispersants which have a considerable effect on the quality and performance of cement compositions.
Workability and consistency of cement compositions reduce gradually with the elapse of time after preparation because of the reaction of hydration of cement with water, for example. This results in a loss in workability, typified by a slump loss of concrete. The magnitude of this slump loss increases in proportion as the unit water content of the cement composition decrease, and as the water reducing ratio in the cement composition increases in conformity with the recent trend. The slump loss of this nature in fresh concrete constitutes a major cause for the limitation imposed on the time allowed for transportation, the alteration of quality and the impairment of formability owing to the waiting time at the site of placement, and such troubles as decline of durability originating as from cold joints. At a factory for the manufacture of secondary concrete products, when the forced transfer of a cement composition with a pump is temporarily suspended and then resumed, the slump loss induces such accidents as a sudden increase in the pressure of forced transfer and a block in the operation of the pump. When the cement composition is placed in a mold and then such a work of formation as compaction is delayed for some reason, the slump loss may result in incomplete placement. At places such as a factory for the preparation of fresh concrete and a factory for the manufacture of secondary concrete products, therefore, the slump loss poses an important task awaiting solution for the quality control of a cement composition and the improvement of work execution.
Various admixture makers, therefore, have been energetically pursuing the development of a so-called high range, air-entraining, water-reducing agent which possesses a high water-reducing property, suffers only a small slump loss, and may be added to the cement composition at a factory for the preparation of fresh concrete. At present, the high range, air-entraining, water-reducing agents of naphthalene type, aminosulfonic acid type, and polycarboxylic acid type are available. An example of a polycarboxylic acid type high range, air-entraining, water-reducing agent is the copolymer which is produced by introducing a polyalkylene glycol mono (meth) acrylic ester type monomer and a (meth)acrylate type monomer together with a monomer copolymerizable with these monomers at a specific ratio as disclosed in JP-B-59-18,338. Other examples include the copolymer which is obtained by polymerizing a polyalkylene glycol diester type monomer having an unsaturated bond and a monomer having a dissociating group as disclosed in JP-A-5-238,795, and the copolymer of a polyalkylene glycol type monomer with an unsaturated bond as an essential component and another specific monomer as disclosed in JP-A-8-12,396. Though these polycarboxylic acid type high range, air-entraining, water-reducing agents have high water-reducing properties and slump-retaining function, they are still not completely satisfactory. Various methods have been proposed for imparting to cement compositions an improved capacity for preventing slump loss. JP-A-54-139,929, for example, teaches a method for preventing slump loss by causing a granular naphthalenesulfonic acid-formalin condensate to be gradually dissolved in a cement composition. JP-A-60-16,851 discloses a method for preventing slump loss by causing a granular copolymer of an olefin with an ethylenically unsaturated dicarboxylic anhydride to be gradually dissolved by hydrolysis in a cement composition. However, the granular natures of the materials mean that they are not storage stable in dispersion form and their effect is not uniform throughout a cement composition to which they are able. JP-A-63-162,562 proposes a method which comprises causing a cement dispersant to be contained in an organic hydrogel such as polyacrylic acid and ensuring gradual release of the dispersant ultimately in a cement composition. This method, however, suffers problematic stability of separation and precipitation because it requires inclusion of a gel insoluble in water.
When very low water/cement ratio (by weight), in the range of 15 to 40%, is used with a view to enhancing strength, impartation of veritably high flowability to a cement composition is difficult to attain even by the polycarboxylic acid type high range, air-entraining water-reducing agent which is reputed to have the highest water-reducing ability among all the high range, air-entraining water-reducing agents of this class. Further, the cement composition has the problem of acquiring no ample workability because of the large slump loss.
An object of this invention, therefore, is to provide a cement dispersant liberated from the problems mentioned above and endowed with an ideal ability to prevent slump loss, a method for the production thereof, and a cement composition using this cement dispersant.
Another object of this invention is to provide a cement dispersant which imparts lasting excellent flowability even to a cement composition having a water/cement ratio (by weight) in the range of 15 to 40% and a cement composition which contains the cement dispersant.
The object mentioned above is accomplished by (i) a cement dispersant having as a main component thereof a polycarboxylic acid type polymer (A) or salt thereof, wherein the polymer (A) has a weight average molecular weight in the range of 10,000 to 500,000 in terms of polyethylene glycol determined by gel permeation chromatography (hereinafter referred to xe2x80x9cGPCxe2x80x9d), and has a value determined by subtracting the peak top molecular weight from the weight average molecular weight in the range of 0 to 8,000.
In this case, the term, xe2x80x9cpeak top molecular weightxe2x80x9d which is used in this invention represents a molecular weight which corresponds to the highest position of the curve plotted on the tested polymer in the GPC chart.
The object can be also attained by (ii) a cement dispersant set forth in (i) above, wherein the polycarboxylic acid type polymer (A) is obtained by copolymerizing 5 to 98% by weight of an (alkoxy)polyalkylene glycol mono(meth)acrylic ester type monomer (a) represented by the following general formula (1): 
wherein R1 stands for a hydrogen atom or a methyl group, R2O for one species or a mixture of two or more species of oxyalkylene group of 2 to 4 carbon atoms, providing two or more species of the mixture may be added either in the form of a block or in a random form, R3 for a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, and m is a value indicating the average addition mol number of oxyalkylene groups which is an integer in the range of 1 to 100,
95 to 2% by weight of a (meth)acrylic acid type monomer (b) represented by the following general formula (2): 
xe2x80x83wherein R4 stands for a hydrogen atom or a methyl group and M1 for a hydrogen atom, a monovalent metal atom, a divalent metal atom, an ammonium group, or an organic amine group, and 0 to 50% by weight of other monomer (c) copolymerizable with these monomers, provided that the total amount of (a), (b) and (c) is 100% by weight.
This object is further accomplished by (iii) a cement dispersant set forth in (ii) above, wherein the (alkoxy)polyalkylene glycol mono(meth)acrylic ester type monomer (a) is (methoxy)polyethylene glycol mono (meth) acrylate and the (meth)acrylic acid type monomer (b) is (meth)acrylic acid.
This object is further accomplished by (iv) a cement dispersant set forth in (i) or (ii) above, wherein the adsorption ratio of the polymer (A) onto cement particles is less than 60% at room temperature for 5 minutes when added in an amount of 0.2% by weight to that of the cement.
This object is further accomplished by (v) a cement dispersant set forth in (i) or (ii) above, wherein the adsorption ratio of the polymer (A) onto cement particles is not less than 60% at room temperature for 5 minutes when added in an amount of 0.2% by weight to that of the cement.
This object is further accomplished by (vi) a method for the production of a cement dispersant as hereinabove defined, which comprises drip-feeding over time into a reaction vessel containing water a monomer mixture (I) containing an unsaturated carboxylic acid type monomer as an essential component under such conditions that the amount of the monomer mixture (I) used is in the range of 10 to 28% by weight to that of the total starting raw materials, the polymerization being effected at a temperature of not more than the cloud point of the monomer mixture (I), and the neutralization ratio of the monomer mixture (I) being in the range of 0 to 20 mol %, and carrying out a polymerization reaction.
This object is further accomplished by (vii) a method for the production of a cement dispersant as hereinabove defined, which comprises drip-feeding over time into a reaction vessel containing water a monomer mixture (I) containing an unsaturated carboxylic acid type monomer as an essential component under such conditions that the neutralization ratio of the monomer mixture (I) being in the range of 0 to 20 mol %, the monomer mixture (I) being mixed with a chain transfer agent prior to a polymerization reaction, and carrying out the polymerization reaction.
This object is further accomplished by (viii) a method for the production of a cement dispersant set forth in (vi) or (vii) above, wherein the monomer mixture (I) comprises 5 to 98%by weight of an (alkoxy)polyalkylene glycol mono(meth)acrylic ester type monomer (a) represented by the following general formula (1): 
wherein R1 stands for a hydrogen atom or a methyl group, R2O for one species or a mixture of two, or more species of oxyalkylene group of 2 to 4 carbon atoms, providing two or more species of the mixture may be added either in the form of a block or in a random form, R3 for a hydrogen atom or an alkyl group of 1 to 5 carbon atoms, and m is a value indicating the average addition mol number of oxyalkylene groups which is an integer in the range of 1 to 100,
95 to 2% by weight of a (meth)acrylic acid type monomer (b) represented by the following general formula (2): 
xe2x80x83wherein R4 stands for a hydrogen atom or a methyl group and M1 for a hydrogen atom, a monovalent metal atom, a divalent metal atom, an ammonium group, or an organic amine group,
and 0 to 50% by weight of other monomer (c) copolymerizable with these monomers, provided that the total amount of (a), (b) and (c) is 100% by weight.
This object is also accomplished by (ix) a cement composition comprising at least cement, water, and a cement dispersant, characterized by containing the cement dispersant set forth in (i) or (ii) above.
The cement dispersant of this invention is excellent in water-reducing ability and is free from the slump loss which is the problem suffered by the conventional high range, air-entraining, water-reducing agent. The cement dispersant, therefore, permits a cement composition such as concrete to be kneaded at a high water-reducing ratio, slump loss is minimal, and, many of quality control and workability problems of the prepared concrete may be overcome. Further, the cement dispersant set forth in (iv) above has a much improved ability to prevent slump loss, and the cement dispersant set forth in (v) above can maintain sufficient flowability in the cement composition having an extremely low water/cement ratio.
The method for the production of a cement dispersant of this invention permits the preparation of a cement dispersant which is very effective in the prevention of slump loss and which has considerable water-reducing capability, thus providing many economic advantages.
The cement composition of this invention far excels the conventional cement compositions in the slump-retaining time and water-reducing ability.